Many autonomous devices that move around an area in a random or planned coverage path include navigation sensors such as cliff sensors that prevent the device from driving over a ledge such as, for example, stairs. A known cliff sensor and its method of operating are disclosed in U.S. Pat. No. 6,594,844, the entire disclosure of which is incorporated herein by reference. A schematic diagram of an exemplary cliff sensor is illustrated in FIG. 1. Many known cliff sensors include an infrared light emitting diode (IR-LED) emitter E that emits a beam from a bottom surface of the device onto the surface over which the device travels, most commonly at a predetermined angle. In turn, the surface can reflect or scatter the light upward, and the light can be fanned out over many three-dimensional angles, but with a high concentration of energy in the beam angle that is equal in size, but opposite in sign, to the IR-LED's angle of emission. A companion detector D (also referred to herein as a receiver), such as a photo-transistor that can be oriented to receive reflections from the illuminated zone, is provided to detect a portion of the scattered energy and send information regarding detected energy (e.g., the existence of the detected energy) to the device's controller to let the controller know that a cliff is not imminent. If no reflection is detected by the receiver, the controller can conclude that there is no surface over which to reflect and thus over which to travel, and can then halt or back away from the threat of falling.
Certain cliff sensors in autonomous devices direct the IR-LED beam onto the surface at an angle of, for example, about 20° to 30° from the vertical, and the receiver can be aimed at the intended illuminated spot at a slightly different angle, with both angles being in a common plane. An exemplary embodiment of a cliff sensor on a robotic cleaning device detecting the presence of a floor under the device is illustrated in FIG. 2A. One skilled in the art will understand that emitted light will be scattered to a greater extent in more reflective surfaces, despite the simplified illustration of FIG. 2A.
Because the cliff sensor works on the notion that a “cliff’ exists when no reflection of the IR-LED beam is received, there can be occasions when no cliff exists, but the environment, surface composition, or surface geometry interferes with reflection of the IR-LED beam back to the receiver, causing false detection of a cliff and inappropriate halting or reversing of the autonomous device. The environment can effect cliff sensor operation when it contains too much ambient light. Surface composition that can interfere with proper cliff sensing includes dark or black carpeting, which absorbs light and thereby can prevent sufficient reflection of light back to the cliff sensor. One example of a surface geometry that interferes with reflection of the IR-LED beam back to the receiver includes certain 30°-60° surfaces encountered by the cliff sensor, particularly when the surface is highly reflective and thus scatters emitted light such that light is not concentrated at the cliff sensor detector to a suitable degree. While such inclined surfaces may not always interfere with accurate cliff sensor detection, they have the potential to interfere therewith.
An exemplary embodiment of a cliff sensor on a robotic cleaning device failing to detect the presence of an inclined surface in front of the device is illustrated in FIG. 2B. As can be seen, the surface in FIG. 2B is inclined at an angle of about 45° with respect to the horizontal. As can be seen, because the beam angle is emitted at, for example 20°, the beams therein are reflected from the inclined surface such that they are not detected by the detector. Thus, the controller may inappropriately halt or reverse the device, believing that a cliff exists. Further, the shinier (more reflective) the inclined surface is, the more the emitted beam is scattered and the less likely it is that the detector will receive enough emitted beam to properly move forward over the surface. An example of a shiny, inclined surface that can be encountered by a floor cleaning robot is a threshold or transition plate, as commonly used to transition between different types of flooring. One skilled in the art will understand that emitted light will be scattered to a greater extent in more reflective surfaces, despite the simplified illustration of FIG. 2B.
Docking stations are known to be used for, e.g., guiding, receiving, and/or charging autonomous devices such as robotic cleaning devices. Docking stations typically provide charging contacts to which contacts on the autonomous device connect so that a power source (e.g., a battery) on the autonomous device can be recharged. Docking stations commonly rest on the floor and provide the charging contact on a raised surface, as shown on FIG. 3.
Docking station design, for example combining aesthetic considerations with functional requirements, may dictate that certain surfaces such as exterior walls supporting the raised surface be inclined at an angle of between 30° to 60° which, as stated above, can cause inappropriate halting and/or reversing of the autonomous device, particularly if such inclined surfaces are highly reflective.